Pressure vessels are commonly used for containing a variety of fluids under pressure, such as storing hydrogen, oxygen, natural gas, nitrogen, propane and other fuels, for example. Suitable container materials include laminated layers of wound fiberglass filaments or other synthetic filaments bonded together by a thermal-setting or thermoplastic resin. A polymeric or other non-metal resilient liner or bladder often is disposed within the composite shell to seal the vessel and prevent internal fluids from contacting the composite material. The composite construction of the vessels provides numerous advantages such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These attributes are due to the high specific strengths of the reinforcing fibers or filaments that are typically oriented in the direction of the principal forces in the construction of the pressure vessels.
FIGS. 1 and 2 illustrate an elongated pressure vessel 10, such as that disclosed in U.S. Pat. No. 5,476,189, which is hereby incorporated by reference. Vessel 10 has a main body section 12 with end sections 14. A boss 16, typically constructed of aluminum, is provided at one or both ends of the vessel 10 to provide a port for communicating with the interior of the vessel 10. The vessel 10 is formed from an inner polymer liner 20 covered by an outer composite shell 18. In this case, “composite” means a fiber reinforced resin matrix material, such as a filament wound or laminated structure. The composite shell 18 resolves all structural loads.
Although the liner 20 provides a gas barrier under typical operating conditions, the design of a pressure vessel 10 of this type produces a phenomenon wherein gas diffuses into the liner 20 under pressurization. When depressurization of the vessel 10 occurs, this gas diffuses into the space between the liner 20 and the composite shell 18. A pocket of gas may thereby be formed, forcing the liner 20 to bulge inward. At low pressure, laminate strain in the composite shell 18 is low, and microcracks in the shell 18 close up, forming a seal; when a higher pressure is reached, those microcracks open up again, thereby allowing expulsion of the trapped pocket of gas. Thus, when the vessel 10 is re-pressurized, pressure builds up against liner 20, pushing against the trapped gas pocket, making the bulge in the liner 20 smaller until the gas is ultimately expelled through the composite shell 18 to the atmosphere. Such expulsion of gas through shell 18 may occur in a short time interval and can cause a significant concentration of gas to become present in the surroundings of the vessel 10. This may set off a leak detector around the vessel 10, when actually there is no steady leak from the vessel 10.